A nonaqueous electrolyte battery (for example, a lithium-ion battery) having a high energy density has attracted attention as a large-sized and large-capacity power source used in an electric vehicle (EV), a hybrid vehicle (HEV), an electric motorbike, and a forklift or the like. The lithium-ion battery is developed to achieve a bigger size and larger capacity while also considering long life and safety or the like. As a large-capacity power source, a battery pack, which houses many batteries connected in series or in parallel so as to increase the electric driving power, had been developed.
Particularly, for a nonaqueous electrolyte battery, aluminum substrate is often used as a positive electrode current collector since aluminium has oxidation resistance.
In recent years, a battery including lithium titanate as a negative electrode has been developed. Such battery including lithium titanate in the negative electrode has recently utilized an aluminum substrate as the negative electrode current collector.
On the other hand, a plurality of conductive current collecting tabs are often extending from an electrode to achieve high output. Ultrasonic joining is suitable in order to join the plurality of current collecting tabs together, and to join the plurality of current collecting tabs to a lead or the like connected to the outside of a battery container. Regarding the joining by, for example, laser, it is difficult to join a plurality of metal plates together to produce one member. Furthermore, when current collecting tabs containing an aluminum material are subjected to resistance welding, the aluminum material tends to be scattered or flung off at the moment of the joining. This is because an oxide film formed on the surface of an aluminum material has a high melting point although aluminum itself has a low melting point and a low boiling point.
However, when the lead attached to a lid and the current collecting tab are subjected to ultrasonic joining, and the ultrasonic wave generated during welding is transmitted to the lid, a gas-relief vent provided in the lid may be broken or deteriorated. Therefore, if no ingenuity is utilized during the ultrasonic joining, the reliability of the gas-relief vent may be decreased.
For this reason, a means for attenuating the transmission of such ultrasonic wave is performed by making a thin portion in a lead part.